Often they have to do this. Most places that you would inspect have bio-security, safety, or other policies that need to be followed for good reasons. The first step of any inspection needs to be find out what those policies are and follow them. Generally the policies are reasonable and based off of sound science, but they are just different enough from place to place that you need to check. Often the place you are inspecting will arrange someone who knows the policies to go with you - but they need to ensure that person isn't on vacation (or otherwise doing something that must be done). Of course checking in like that tips people off that you are going to inspect.
If you enter a barn your clothing needs to be disinfected, you need to shower, and you can't enter any other barn for a day (most farms with this policy have more than one barn so the inspection will be several days or you needs lots of inspectors). A forklift will not see you, so you need to understand where the safe places are so the forklift doesn't run you over.
I am for surprise inspections. However there is very good reason you cannot have them despite the value.